Experimental Investigation of $\pi^+\pi^-$ and $K^+K^-$ atoms

Leonid Afanasyev, Dirac Collaboration
2019 Proceedings of The 39th International Conference on High Energy Physics — PoS(ICHEP2018)   unpublished
The adapted DIRAC experiment at the CERN PS accelerator observed for the first time longlived hydrogen-like π + π − atoms, produced by protons hitting a beryllium target. A part of these atoms crossed the gap of 96 mm and got broken up in the 2.1 µm thick platinum foil. Analysing the observed number of atomic pairs, n L A = 436 +157 −61 tot , the lifetime of the 2p state is found to be τ 2p = ( 0.45 +1.08 −0.30 tot ) · 10 −11 s, not contradicting the corresponding QED 2p state lifetime τ QED 2p
more » ... = 1.17 · 10 −11 s. This lifetime value is three orders of magnitude larger than our previously measured value of the π + π − atom ground state lifetime τ = ( 3.15 +0.28 −0.26 tot ) · 10 −15 s. Further studies of long-lived π + π − atoms will allow to measure energy differences between p and s atomic states and so to determine ππ scattering lengths with the aim to check QCD predictions. At the same setup, there were identified more than 7000 K + K − pairs with effective mass less than 2M K + 5 MeV. In the distributions of K + K − pairs there is a strong signature of the Coulomb enhancement: the number of pairs increases with decreasing of the relative momentum in the pair c.m.s. The observed number of K + K − pairs with small relative momentum will allow us to evaluate for the first time the number of the produced K + K − atoms.
doi:10.22323/1.340.0453 fatcat:nq7idcw4djdxfe6iboba6j2564